It is known that processes for producing phosphoric acid include a dry process and a wet process. In the manufacture of phosphoric acid by the dry process, phosphate rock is reduced in an electric furnace, and the resulting yellow phosphorus is burnt into diphosphorus pentoxide, which is hydrated to obtain phosphoric acid. In the wet process, phosphate rock is decomposed with sulfuric acid, and produced calcium sulfate is separated to obtain dilute phosphoric acid, which is then concentrated into high concentration acid. By whichever process it is produced, phosphoric acid is required to have high purity in electronics applications.
Phosphoric acid from the dry process (dry-process phosphoric acid), which is produced via yellow phosphorus, contains less impurity of phosphate rock origin and is said to have better quality than that from the wet process. High purity phosphoric acid from the dry process with a purity ranging 75% to 85% by weight is used in etching of semiconductors. If phosphoric acid for this application contains much antimony or arsenic as impurity metals, fine particles of these impurity metals tend to remain on silicon wafers depending on equipment for the semiconductor fabrication. Such impurity metal particles can cause trouble in the following step or necessitate an extra step for washing the particles off the wafer. Hence, it has been demanded to provide phosphoric acid with still higher purity.
A process for purifying phosphoric acid of 75% to 85% by weight concentration as H3PO4 containing antimony and arsenic as impurity metals to make it acceptable for electrical semiconductor use is known (see Patent Document 1), in which hydrogen sulfide is added to the phosphoric acid to precipitate the impurity metals in the acid, and the precipitate is separated from the treated acid at a temperature of at least 60° C. According to the purification process, phosphoric acid after the purification having a concentration of 85% as weight percent H3PO4 contains 13 to 20 ppm of antimony and 0.02 to 0.1 ppm of arsenic.
A method of removing arsenic by bringing phosphoric acid into contact with a hydrogen halide is also known (see Patent Document 2). The method is capable of reducing arsenic in phosphoric acid to 1 ppm or less. The publication also says that, when the contact between phosphoric acid and a hydrogen halide is in the presence of a compound capable of generating a hydrogen halide under an acidic condition, the arsenic removing effect increases to reduce the arsenic content in the resulting phosphoric acid to 0.1 ppm or lower. According to the disclosure, however, the arsenic content of phosphoric acid (P2O5 concentration: 65%) reached in the working example is only about 0.07 to 0.8 ppm, which level does not sufficiently meet the demand for high purity phosphoric acid. Moreover, the method needs hydrogen peroxide, and iron chloride or tin chloride must be used before bubbling hydrogen chloride in order to achieve sufficient removal of arsenic.
Apart from the above-described methods, a technique of purifying phosphoric acid by crystallization is widely known (see e.g., Patent Document 3). This patent document discloses a process in which a series of purification operations consisting of crystallization, separation from the mother liquor, and melting is repeated three times to obtain desired phosphoric acid purity. Basic physical property data necessary in purifying phosphoric acid by crystallization are known (see Non-Patent Document 1), including saturation solubility, correlation between supersaturation and crystal growth rate, hygroscopicity of phosphoric acid hemihydrate crystals, and crystal sedimentation rate in phosphoric acid. There are also reports on specific applications to a fluidized bed crystallization system (see Non-Patent Document 2). However, there is no clear description about the purity of phosphoric acid obtained by crystallization operation. In addition, crystallization operation has a disadvantage of increase in cost.
Patent Document 1: JP-A-1-131009
Patent Document 2: WO 00/40507
Patent Document 3: JP-B-44-14692
Non-Patent Document 1: Oimura et al., Toyo Soda Kenkyu Hokoku, 10, 2, 21 (1966)
Non-Patent Document 2: Aoyama et al., Proceedings of a Conference of Industrial Crystallization, pp. 413-420 (1976)